Vehicles in the form of riding mowers and tractors having a traditional mechanical steering system with a steering wheel controlled by a seated operator are in widespread use. While this type of steering system generally provides an acceptable level of steer wheel traction and dynamic stability during regular turning at low speeds, problems occur during relatively sharp turns if the vehicle speed is relatively high and left unchecked. This is especially true in vehicles equipped with modern, Ackerman-type steering systems that allow the steerable wheels to pivot different amounts up to almost 90° in either direction (180° total) to achieve particularly tight turns.
For example, in the case of a self-propelled lawn mower, it is difficult if not impossible at high speeds to make the tight turn necessary to mow around a small round or circular object, such as a pole or shrub, without the operator actively reducing the vehicle speed. Even at moderate speeds, a sudden, sharp turn may also result in a lifting of the wheels from the ground and a concomitant loss of traction. This is deleterious, in that it allows the vehicle to essentially slide out of the turn and effectively miss the intended path of travel, which can not only be frustrating, but may also damage the turf. The lifting of the wheels also reduces dynamic stability and, hence, poses safety concerns. These concerns led the American National Standards Institute (ANSI) to promulgate standards regarding the maximum amount of wheel lift permitted during a turn for self-propelled, rider-operated lawn mowers. To meet the standards, it is thus desirable to automatically reduce the speed of the vehicle as a turn is made in an effort to reduce the amount of wheel lift or eliminate it altogether.
In the past, others have proposed control systems that automatically slow a vehicle during turns in an effort to address and overcome the foregoing problems. One such system is disclosed in Toro's U.S. Pat. No. 6,092,617 to White, III. et al., the disclosure of which is incorporated herein by reference. While this “slow-in-turn” system may achieve the desired result, it is terribly complicated in design and, thus, expensive to manufacture, install, and maintain. The part of the system providing the slow-in-turn capabilities is also not well-suited for retrofitting on most existing vehicles, since it primarily applies to a specialized type of drive system. The particular embodiments of the system disclosed in this patent also slow the speed of the vehicle regardless of the degree of turning. However, for a certain range of turns, a reduction in vehicle speed is simply not required to maintain traction and achieve dynamic stability.
Another vehicle control system with slow in turn characteristics is described in U.S. Pat. No. 4,572,310 to Peter, which is also incorporated herein by reference. However, it basically applies to machines with drive wheels controlled by individual clutches. Consequently, it is not well-suited for application to the current designs of the majority of lawn mowers, tractors, and similar vehicles that do not have such clutches.
A vehicle having extremely tight turning characteristics is described in my U.S. Pat. No. 6,185,920, which is also incorporated herein by reference. In view of the tight turning radius that can be achieved by this vehicle, it can especially benefit from the steer traction and dynamic stability enhancement provided by a slow-in-turn system. This patent proposes accomplishing the enhancement by providing individual left and right brakes for the drive wheels. Moreover, the speed of the individual driven wheels can be manually controlled to slow the vehicle down during a turn to improve both tracking and traction. While these arrangements thus provide beneficial slow-in-turn capabilities, both require active or independent manual input from the operator.
Accordingly, a need is identified for an improved vehicle control system with slow-in-turn capabilities. The system would automatically slow the vehicle during turns made beyond or outside a predetermined range, and especially during the sharpest of turns. The automatic nature of the system means that independent manual input by the operator would be unnecessary to make the speed adjustment. The system would also be simple and robust in both design and operation, thus keeping the manufacturing, installation, and maintenance expense at a minimum, yet without sacrificing functionality. This simplicity would also allow the system to be easily adapted for use with or retrofitted onto many types of existing self-propelled, rider-operated vehicles, such as riding lawn mowers, without significant cost or effort.